In one class of fluid systems, such as common rail fuel systems for internal combustion engines, a variable displacement pump provides pressurized fluid to a common rail, which then transmits the pressurized fluid to a plurality of fuel injectors. These variable displacement pumps maintain the common rail at a desired pressure by utilizing a spill valve to controllably displace fluid to the common rail or to a low pressure reservoir. For example, in these pumps, when a piston is undergoing a pumping stroke within a chamber, pressurized fluid displaced from the chamber either passes through a check valve to the common rail or through the spill valve to the low pressure reservoir. If the spill valve is open, then the pressurized fluid passes through the spill valve and into the low pressure reservoir, which is the path of least resistance. However, if the spill valve is closed, pressure inside of the chamber quickly increases and the pressurized fluid is forced through the check valve and into the common rail. Accordingly, by controlling the frequency at which the spill valve cycles between open and closed positions, the plump selectively provides pressurized fluid to the common rail for maintaining the stability of the pressure therein.
Spill valves used in known variable displacement pumps typically include a valve stem connected to a solenoid-operated armature and extending through a middle passage of a valve block. A valve seat can be formed in one end of the valve block for receiving a sealing surface formed on one end of the valve stem. A contact surface, located in a facing relationship with the armature, can be formed on the other end of the valve block. The solenoid energizes and de-energizes for moving the armature out of and in to contact with the contact surface and for seating and unseating the sealing surface of the valve stem in and out of the valve seat. Oftentimes, however, these known pumps include an annular spacer located between the contact surface of the valve block and the armature. The spacer is typically fixed to, and movable with, the armature. As such, the spacer, not the armature, makes contact with the contact surface of the valve block.
The spill valve is typically open when the solenoid is de-energized. In this position, the armature and the annular spacer rest against the contact surface of the valve block and the valve assembly projects a distance out of the other side of the valve block such that an opening exists between the sealing surface and the valve seat. The spill valve is typically closed when the solenoid is energized. When energized, the solenoid causes the armature, including the spacer, to move upward, away from the contact surface of the valve block. This upward movement of the armature separates the spacer from the contact surface and retracts the valve assembly into the valve block causing the sealing surface to seat in the valve seat. When the solenoid is de-energized, therefore, the spill valve is open and no fuel is delivered to the rail. On the other hand, when the solenoid is energized, the spill valve is closed and fuel is delivered to the rail.
While these known variable discharge pumps are suitable for many purposes, they are not always well suited for use with modern hydraulically actuated fuel systems, which require fluid delivery to the rail to be varied with high precision and with rapid response times measured in microseconds. For example, these known variable discharge pumps may not be well suited for use with modern fuel systems in cold weather conditions, such as when an engine is undergoing a cold start. This is because, at cold temperatures, the pressurized fluid becomes viscous causing a sticking phenomenon occur where the spacer contacts the contact surface of the valve block. This sticking phenomenon inhibits or delays the ability of the armature, including the spacer, to break free from the contact surface of the valve block. This sticking phenomenon is sometimes referred to as stiction, which may be caused by a relatively thin but highly viscous fluid layer between the spacer and the contact surface. Stiction, by decreasing the response time of the armature, inhibits the pump's ability to control the frequency at which the spill valve cycles between open and closed positions. Accordingly, stiction may result in rail pressure instability.
It should be appreciated that the foregoing background discussion is intended solely to aid the reader. It is not intended to limit the disclosure or claims, and thus should not be taken to indicate that any particular element of a prior system is unsuitable for use, nor is it intended to indicate any element to be essential in implementing the examples described herein, or similar examples.